Research Areas > Stormwater > Stormwater Plume Detection

Project: Stormwater Plume Detection

Background and Objectives

Satellite information can be a cost-effective way to make synoptic water quality assessments. Detection of ocean and other characteristics by satellite can be used to estimate when and where high concentrations of contaminants occur. This is because concentrations of contaminants in stormwater plumes (e.g., fecal indicator bacteria) are related to salinity, which, in turn, may be related to ocean color. Satellite information can be especially useful to coastal managers and governing agencies that are required to perform ongoing monitoring, and to post beach closure warnings if contaminants like fecal indicator bacteria are found in levels exceeding state standards. The goal of this research is to develop the ability to routinely detect and characterize stormwater runoff plumes in the Southern California Bight.

Status

This study was conducted from 2005 to 2007.

Methods

Stormwater plumes in the southern California coastal ocean were detected by MODIS-Aqua satellite imagery collected during the Bight’03 Regional Water Quality Program surveys in February-March of 2004 and 2005. MODIS imagery was processed using a combined near-infrared/shortwave-infrared (NIR-SWIR) atmospheric correction method, which substantially improved normalized water-leaving radiation (nLw) optical spectra in coastal waters with high turbidity. Plumes were detected using a minimum-distance supervised classification method based on nLw spectra averaged within the training areas, defined as circular zones of 1.5-5.0-km radius around field stations with a surface salinity of S<32.0 (“plume”) and S>33.0 (“ocean”). To assess the accuracy of plume detection, stations were classified into “plume” and “ocean” using two criteria: 1) “plume” included the stations with salinity below a certain threshold estimated from the maximum accuracy of plume detection; and 2) bacterial counts in “plume” exceeded the California State Water Board standards.

Findings

• The plume optical signatures (i.e., the nLw differences between “plume” and “ocean”) were most evident during the first two days after the rainstorms.

• The salinity threshold between “plume” and “ocean” was estimated as 32.2.

• The total accuracy of plume detection in terms of surface salinity was not high (68% on average), seemingly because of imperfect correlation between plume salinity and ocean color.

• The accuracy of plume detection in terms of bacterial exceedances was even lower (64% on average), resulting from low correlation between ocean color and bacterial contamination.

• In most southern California coastal areas, the zones of bacterial contamination were much smaller than the areas of turbid plumes. An exception was the plume of the Tijuana River, where the zone of bacterial contamination was comparable with the zone of plume detected by ocean color.

Partners

NOAA/NESDIS Center for Satellite Applications and Research
Department of Biological Sciences, University of Southern California
Aquatic Bioassay and Consulting Laboratories
Orange County Sanitation District
USGS Coastal and Marine Geology Program